How do you figure the gallons in a well?
Three numbers from the well record do it: total depth, static water level (the depth down to the water), and casing diameter. Standing water is total depth minus water level. A round casing holds 0.0408 × diameter² gallons per foot, so a 6-inch well holds about 1.47 gallons per foot. We round a 6-inch well up to 1.5 so the dose is never light.
No record? Most states keep one. Find your state's well records or see typical well depths.
When does a well pro come before treatment?
If a test found E. coli, call a well pro first. E. coli means something is getting into the well, and a pro should find the cause and fix it before anyone treats the well. Our treatment kit is not the fix for E. coli. After a flood, have a pro check the well cap, casing, pump and wiring before you turn the pump on. Coliform without E. coli, work on the well, and routine care are the cases to treat yourself, then test again about a week later.
Not sure of the casing size? The casing is the pipe that sticks up out of the ground. A driven point well is about 2 inches across, and most drilled wells are 5 or 6 inches. A dug or bored well can be 2 to 3 feet wide.
Worked example
This is Montana DEQ's own example: a 6-inch well, 150 ft deep, with water at 50 ft.
- Standing water: 150 − 50 = 100 ft.
- Gallons in the well: 100 ft × 1.5 gal per ft = 150 gallons.
- Add 100 gallons for the plumbing: 250 gallons to treat.
- Bleach at 3 pints of 5.25% per 100 gallons: 7.6 pints (about 1 gallon).
- The same well with our treatment: pouches holding 15 grams for 155 gallons at the standard dose, $329 for the kit.
What chlorine level disinfects a well?
Montana DEQ calls for a 200 ppm chlorine solution held in the well and plumbing for at least 2 hours, overnight if you can. The Minnesota Department of Health gives 50 to 200 ppm and says not to go above 200 ppm. The calculator uses 200 ppm and plain, unscented household bleach.
What do you do after the bleach goes in?
Circulate the water back down the well with a hose, run each faucet until it smells of chlorine, let it stand, then flush it outside and retest for bacteria about a week later. Our step-by-step well treatment guide covers each step, and the kit page shows what ships in the box.
Questions
How much bleach do I need to shock a well?
Montana DEQ uses 3 pints of 5.25% household bleach per 100 gallons of water in the well and plumbing, which makes about a 200 ppm chlorine solution. Add 100 gallons to the well's own gallons for the plumbing, pressure tank and water heater. A 6-inch well with 100 feet of standing water holds about 150 gallons, so it needs about 7.6 pints for 250 gallons.
How many gallons of water are in my well?
Subtract the static water level from the total depth to get feet of standing water. Multiply by 0.0408 times the casing diameter in inches squared. A 6-inch casing holds about 1.47 gallons per foot (we round it up to 1.5), a 4-inch casing about 0.65, and a 2-inch casing about 0.16.
What chlorine level should I use to disinfect a well?
Montana DEQ calls for a 200 ppm chlorine solution in the well and plumbing for at least 2 hours, and overnight is better. The Minnesota Department of Health gives a range of 50 to 200 ppm and says not to go above 200 ppm. This calculator uses 200 ppm.
What if my bleach is 6% or 8.25% instead of 5.25%?
Stronger bleach takes less. The amount is the target ppm times the gallons, divided by the strength in percent times 10,000. For 100 gallons at 200 ppm that is about 3 pints of 5.25% bleach, 2.7 pints of 6%, or 1.9 pints of 8.25%. Pick your strength in the calculator and it does this for you. Use plain, unscented bleach.
How long does the chlorine stay in the well?
Montana DEQ says to hold the chlorinated water in the well and plumbing for at least 2 hours, preferably overnight, then flush it out through an outside faucet until the strong chlorine smell is gone. Retest the water for bacteria about one week later.
How is the Save My Well treatment sized?
Our kit uses chlorine dioxide pouches, not bleach. We size it to your well's gallons plus 5 gallons for the line and hose, at the standard 20 ppm dose inside the maker's published 20 to 50 ppm range, and round up to whole 5-gram units. Your state sets which pouch sizes ship. Every constant in that math is listed with its source on our sources page.
Every number in our own dosing math is on the sources page.
Add this calculator to your site
Health departments, extension offices and well pros may embed this calculator for free. Paste this code into your page. Please keep the "Calculator by Save My Well" line under it.
<iframe src="https://www.savemywell.com/embed/well-disinfection-calculator" title="Well disinfection calculator" width="100%" height="900" style="border:0;max-width:640px" loading="lazy"></iframe>
<p style="font-size:13px"><a href="https://www.savemywell.com/well-disinfection-calculator">Calculator by Save My Well</a></p>Sources
- Montana DEQ, Fact Sheet SWP-101: Disinfecting your water supply (Feb 2019)
- Minnesota Department of Health: Well Disinfection
- CDC: How to Disinfect Wells After an Emergency
- Selectrocide product label (EPA Reg. No. 74986-5)
Follow all directions on the product label of whatever you put in your well.